Improve async fn error message with link to workaround

Author: dtolnay

syntax/parse.rs

@@ -373,6 +373,12 @@ fn parse_extern_fn(
             "variadic function is not supported yet",
         ));
     }
+    if foreign_fn.sig.asyncness.is_some() {
+        return Err(Error::new_spanned(
+            foreign_fn,
+            "async function is not directly supported yet, but see https://cxx.rs/async.html for a working approach",
+        ));
+    }
 
     let mut doc = Doc::new();
     let mut cxx_name = None;

tests/ui/async_fn.stderr

@@ -1,22 +1,5 @@
-error[E0308]: mismatched types
- --> $DIR/async_fn.rs:4:18
+error: async function is not directly supported yet, but see https://cxx.rs/async.html for a working approach
+ --> $DIR/async_fn.rs:4:9
   |
 4 |         async fn f();
-  |                  ^
-  |                  |
-  |                  expected `()`, found opaque type
-  |                  possibly return type missing here?
-...
-8 | async fn f() {}
-  |              - the `Output` of this `async fn`'s found opaque type
-  |
-  = note: expected unit type `()`
-           found opaque type `impl Future`
-help: consider `await`ing on the `Future`
-  |
-4 |         async fn f.await();
-  |                   ^^^^^^
-help: try adding a semicolon
-  |
-4 |         async fn f;();
-  |                   ^
+  |         ^^^^^^^^^^^^^